1. Field of the Invention
This invention relates to the measurement of the radiation characteristics of microwave and millimeter-wave antennas, and more particularly to a near-field measurement system which measures the radiating near-field of the antenna under test (AUT) on a planar bi-polar data collection grid.
2. Description of the Related Art
Much attention has been given to the process of measuring the radiating near-field of microwave antennas as a means to determine the far-field radiation pattern and to perform antenna diagnostics. In a typical near-field antenna measurement system the field (both amplitude and phase) of the AUT is measured with a field probe over a virtual surface. This data is then transformed to the far-field antenna radiation pattern by computer processing to evaluate antenna performance. Alternatively, the near-field data may be transformed back toward the AUT to provide information about the aperture fields and to perform antenna diagnostics.
A typical near-field measurement range consists of three primary subsystems: a computer, a robotic positioner and a microwave source/receiver. The computer provides the user interface and controls the positioner and source/receiver. In addition it commands the robotic positioner which moves the test antenna, a field probe, or both over the desired virtual surface. The microwave subsystem measures the amplitude and phase of the fields surrounding the antenna during this scanning process. Reciprocity considerations allow either the probe or AUT to transmit.
The type of near-field scanner is determined by the measurement surface scanned by the field probe. Realizable surfaces include planes, circular cylinders and spheres which correspond to three major near-field scanning systems: planar, cylindrical and spherical. These systems each have certain advantages depending upon the antenna to be measured. The simplest and most common scanning systems are of the planar type. When using the planar method, it is assumed that the AUT radiates predominantly in a single direction and, therefore, planar scanners are popular for the measurement of reflector antennas and planar arrays. The near-field to far-field transformation is a simple two-dimensional Fourier transform of the near-field data. This transform is calculated by computer, often using a fast Fourier transform (FFT) algorithm. The field is typically sampled every half wavelength to satisfy the Nyquist sampling criteria. An excellent review of current near-field measurement technology can be found in the special issue of the Institute of Electrical and Electronics Engineers, Antennas and Propagation Journal, volume AP-36, Jun. 1988.
In the past, planar near-field scanners were categorized as being either plane-rectangular or plane-polar. For the plane-rectangular case, the probe is typically driven linearly in a raster scan over a plane in front of the antenna. The raster scan causes the measurement points to lie on a rectangular grid. The plane-polar approach typically uses a linearly scanning probe over a rotating AUT. For the plane-polar technique, data is collected on concentric rings centered at the middle of the scan plane. The linear motion of the probe causes the measurement points to lie on radial lines.
The above mentioned planar near-field measurement systems, namely plane-rectangular and plane-polar, both require the mechanical positioning of the probe along linear paths. The present invention is a planar near-field scanner which offers another approach requiring only rotational mechanical motion. The mechanical implementation of rotational motion is typically simpler than linear motion, and due to this simplicity the cost of implementing the bi-polar near-field scanner is reduced.